Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
Measurements performed over multiple angles yield information with greater accuracy and precision. In one example, spectroscopic ellipsometry (SE) and spectroscopic reflectometry (SR) systems perform simultaneous measurements across a broad spectrum of illumination wavelengths. However, many existing SR and SE systems acquire measurement signals at one angle of incidence (AOI) at a time. This limits the throughput of such system when multiple AOIs are required to accurately characterize the sample.
In one example, a multi-angle SE instrument available from J. A. Woollam Co., Lincoln, Nebr. (USA), includes mechanisms for rotating the specimen under measurement, elements of the optical system, or both, to sequentially perform measurements at different AOI values. In another example, a multi-angle SE instrument available from KLA-Tencor Corp., Milpitas, Calif. (USA), employs a large numerical aperture (NA) optical system that captures all AOIs of interest simultaneously without moving the specimen or significant portions of the optical system.
In this example, the collection pupil contains the full range of angles reflected by the specimen. Depending on the NA of the system, the full range of angles could range from very small (e.g., collimated) to very large (e.g., greater than five degrees). In existing metrology system the reflected light over the full range of angles is not used to perform a measurement because measurement signal information associated with too many angles of incidence is integrated at the detector. The resulting loss of signal fidelity limits the effectivity of the measured signals. To mitigate this effect, the range of measured AOIs is limited to a few degrees about each nominal AOI that is spectrally measured.
In the multi-angle SE instrument available from KLA-Tencor, mechanical shutters are employed to sequentially measure spectra at one or more AOI sub-ranges. In this example, a specimen is illuminated with a large NA over the full wavelength range of interest and the reflected light is collected by the system optics. The collection pupil contains all the angles of interest, but mechanical shutters or beam blocks are employed to block all collected light except a selected range of AOIs from which light is collected. This selected range remains unblocked and is measured by the measurement sensor. In this approach the optics are maintained in a stationary configuration and AOI selection is achieved with mechanical shutters or masks. Sequential spectral measurements at different AOIs results in extended wafer exposure and overall measurement time. Furthermore, time-dependent effects that manifest on the wafer may be captured by the sequential measurements and negatively impact the measurement results.
In summary, ongoing reductions in feature size and increasing depths of structural features impose difficult requirements on optical metrology systems. Optical metrology systems must meet high precision and accuracy requirements for increasingly complex targets at high throughput to remain cost effective. In this context, speed of data collection and range of angles of incidence have emerged as important factors in the design of optical metrology systems. Thus, improved metrology systems and methods to overcome these limitations are desired.